1. Field of the Invention
The present invention relates a semiconductor device and a method of manufacturing the device, and more specifically, to a semiconductor device with a field-effect transistor having an improved current drivability and a method of manufacturing such a device.
2. Description of the Background Art
As an example of a conventional semiconductor device, a semiconductor device having a field-effect transistor will be described below in relation to the drawings. As seen from FIG. 21, a pair of source/drain diffusion regions 105a, 105b positioned at a prescribed interval are formed on a surface of a silicon substrate 101. On the region of silicon substrate 101 located between the pair of source/drain diffusion regions 105a, 105b a gate electrode 104a is formed, with a gate oxide film 103a formed therebetween. Gates sidewall oxide films 106a, 106b are formed, one on each side surface of gate electrode 104a. A source/drain electrode 107a is formed on the surface of a source/drain diffusion region 105a. Moreover, a source/drain electrode 107b is formed on the surface of a source/drain diffusion region 105b. A gate upper electrode 107c is formed on gate electrode 104a. 
In the above-described manner, the main portion of a semiconductor device having a field-effect transistor is formed on silicon substrate 101. The field-effect transistor is electrically isolated from another field-effect transistor (not shown) by an element isolating oxide film 102 formed in silicon substrate 101.
Now, an example of a method of manufacturing the above-described semiconductor device will be described with reference to the drawings. As shown in FIG. 22, element isolating oxide film 102 is formed on the surface of silicon substrate 101 by trench isolation method. Then, as shown in FIG. 23, a silicon oxide film 103 is formed on the surface of silicon substrate 101 using thermal oxidation method or the like. On silicon oxide film 103, a polysilicon film 104 is formed by CVD (Chemical Vapor Deposition) method or the like. On polysilicon film 104, a photo resist (not shown) is provided, and a photo resist pattern 108 is formed by the use of an appropriate photolithography.
Now, as shown in FIG. 24, using photo resist pattern 108 as a mask, polysilicon film 104 and silicon oxide film 103 are anisotropically etched to form gate electrode 104a and gate oxide film 103a. Thereafter, photo resist pattern 108 is removed.
Next, as shown in FIG. 25, using gate electrode 104a as a mask, an impurity of a prescribed conductivity type is implanted into a surface of silicon substrate 101 using ion implantation method to form a pair of source/drain diffusion region 105a, 105b, respectively. Then, as shown in FIG. 26, a silicon oxide film 106 is formed on silicon substrate 101 to cover gate electrode 104a by CVD method.
Next, as shown in FIG. 27, silicon oxide film 106 is etched anisotropically to form gate sidewall oxide films 106a, 106b, each of which is formed respectively on each side surface of gate electrode 104a. Then, as shown in FIG. 28, silicon is epitaxially grown selectively on gate electrode 104a and source/drain diffusion regions 105a, 105b by epitaxial growth method to form gate upper electrode 107c and source/drain electrodes 107a, 107b, respectively. In this manner, the main portion of the semiconductor device having the field-effect transistor shown in FIG. 21 is completed.
In recent years, miniaturization of field-effect transistors has been promoted in order to keep up with the higher degrees of integration achieved in semiconductor devices. As a field-effect transistor is miniaturized, its gate length is reduced, which leads to a lower threshold voltage, causing the so-called short-channel effect leading to the incorrect operation of the field-effect transistor. Conventionally, in order to prevent the short-channel effect in such a field-effect transistor, the film thickness of the gate oxide film has been reduced, or the depth of a source/drain region (or the depth of junction) has been made smaller. With a smaller depth of the source/drain region, however, the electrical resistance (sheet resistance) in the source/drain region cannot be sufficiently lowered, and the amount of the current flowing through the source/drain region becomes smaller. As a result, problems such as lowering of the current drivability in the field-effect transistor arise, leading to a decreased operation speed. Conventionally, in order to prevent such problems, conductive layers, i.e. source/drain electrodes 107a, 107b, are formed on the surfaces of the source/drain regions to reduce the sheet resistance of the source/drain regions, thereby ensuring the current drivability of the field-effect transistor.
In the above-described semiconductor device, however, source/drain electrodes 107a, 107b were not formed on the portions (extension portions E) located beneath gate sidewall oxide films 106a, 106b on the surfaces of source/drain diffusion regions 105a, 105b. Therefore, it was impossible sufficiently to reduce the sheet resistance of source/drain diffusion regions 105a, 105b in extension portions E. Consequently, further improvement in the current drivability of the field-effect transistor was limited.
The present invention was made to solve the above problems. An object of the present invention is to provide a semiconductor device having an improved current drivability. Another object of the present invention is provide a method of manufacturing such a semiconductor device.
According to one aspect of the present invention, the semiconductor device is provided with a semiconductor substrate having a main surface, an electrode, a pair of conductive regions, and sidewall insulating films. The electrode is formed on the main surface of the semiconductor substrate with an insulating film therebetween. The pair of conductive regions are formed on the semiconductor substrate such that the conductive regions sandwich the electrode from both sides. The sidewall insulating films are formed one on each side surface of the electrode, and recessed portions are formed exposing the main surface of the semiconductor substrate. Further, the pair of conductive regions include impurity regions respectively formed on the main surface of the semiconductor substrate such that the impurity regions sandwich the electrode from both sides, and conductive layers formed on the impurity regions to fill the recessed portions.
According to this construction, a field-effect transistor (simply referred to as a xe2x80x9ctransistorxe2x80x9d below) including an electrode and a pair of conductive regions is formed on the semiconductor substrate. In the transistor, recessed portions exposing the main surface of the semiconductor substrate are formed in the sidewall insulating films provided one on each side surface of the electrode. Moreover, conductive layers in the pair of conductive regions are formed on the impurity regions to fill the recessed portions. Therefore, a conductive layer is also formed between the sidewall insulating film and the impurity region located beneath the sidewall insulating film. As a result, in comparison with the construction of a conventional semiconductor device in which a conductive layer is not formed between the sidewall insulating film and the impurity region, the sheet resistance of the conductive region can be further reduced. Consequently, the amount of current that flows through the conductive regions increases, leading to an improved current drivability of the field-effect transistor as well as an improved operation speed, among others, of the transistor.
The semiconductor substrate, preferably, is a silicon single crystal substrate, and the conductive layer is epitaxially grown silicon or silicon germanium.
In this case, the conductive layer can be easily formed on the impurity region in a self-aligned manner.
In addition, the conductive layer preferably contains a metal. In this case, the sheet resistance of the pair of conductive regions can be further reduced, resulting in an improved current drivability of the transistor.
Further, the semiconductor substrate, preferably, is a silicon substrate, and the conductive layer contains a metal silicide formed by reacting the silicon in the silicon substrate with a metal.
In this case, the conductive layer containing the metal silicide can be easily formed on the impurity region in a self-aligned manner.
In another aspect of the present invention, a method of manufacturing the semiconductor device includes the following steps. An electrode is formed on the main surface of the semiconductor substrate with a first insulating film therebetween. A second insulating film is formed on the semiconductor substrate to cover the electrode. By anisotropically etching the second insulating film, sidewall insulating films are formed one on each side surface of the electrode. A pair of conductive regions are formed on the semiconductor substrate such that the conductive regions sandwich the electrode. The step of forming the sidewall insulating films includes a recessed portion forming step or the step of forming on the sidewall insulating films recessed portions exposing the surface of the semiconductor substrate by removing the portions of the sidewall insulating films in contact with and in the vicinity of the main surface of the semiconductor substrate which were damaged by anisotropic etching. The step of forming a pair of conductive regions includes the step of forming a pair of impurity regions respectively on the main surface of the semiconductor substrate such that the impurity regions sandwich the electrode from both sides, and the step of forming conductive layers electrically connected to the impurity regions upon the main surface of the semiconductor substrate including exposed surfaces to fill the recessed portions.
According to this manufacturing method, a transistor having an electrode and a pair of conductive regions is formed on a semiconductor substrate. In sidewall insulating films provided one on either side surface of the electrode of the transistor, recessed portions exposing a surface of the semiconductor substrate are formed. Portions of the sidewall insulating films in the vicinity of the surface of the semiconductor substrate suffer more damage than other portions from the irradiation of ions or electrons or the like on the surface of the semiconductor substrate upon the formation of the sidewall insulating films by the anisotropic etching of the second insulating film. Since the recessed portions are formed by removing the portions of the sidewall insulating films having suffered more of the damage, the recessed portions can be formed fairly easily. Moreover, by forming the conductive layers to fill the recessed portions, the conductive layers will also be formed between the sidewall insulating films and the impurity regions located beneath the sidewall insulating films. Thus, the sheet resistance of the conductive regions can be further reduced than in the conventional semiconductor device structure having no conductive layer formed between the sidewall insulating films and the impurity regions. As a result, a semiconductor device with a transistor having a greater current drivability can be produced with ease.
As described above, the recessed portions are formed in the portions of the sidewall insulating films which have suffered more damage by anisotropic etching. Degradation in the film quality such as weakened bond strength in the damaged portions is observed in comparison with other portions. Thus, it is desirable to form the recessed portions by vaporizing the damaged portions of the sidewall insulating films by heat treatment. Since the bond strength of the sidewall insulating films is weakened in the damaged portions, the damaged portions are more readily vaporized than other portions through heat treatment, and the recessed portions are formed without difficulty.
In addition, the heat treatment is performed preferably at a temperature of 850xc2x0 C. or above with the degree of vacuum being xc3x9710xe2x88x926 Torr or below. With these conditions, the damaged portions are nearly fully vaporized It is also desirable to form the recessed portions by removing the damaged portions by isotropic etching. The film in the damaged portions is less dense than in other portions, which leads to a faster etching rate. Thus, the recessed portions can be formed without difficulty.
As to the sidewall insulating films, specifically, an insulating film selected from the group consisting of a silicon oxide film, a silicon nitride film, and a silicon oxynitride film may be used.
In addition, the semiconductor substrate is a silicon single crystal substrate. Moreover, the step of forming the conductive layers preferably includes an epitaxial growth step or the step of forming the conductive layers by silicon epitaxial growth method.
In this case, the conductive layers can be formed easily and in a self-aligned manner on the impurity regions formed on the surface of the semiconductor substrate.
The epitaxial growth step preferably includes the step of forming the conductive layers such that voids do not form between the growing silicon and the sidewall insulating films and that faceting does not occur at an edge of the growing silicon on the main surface of the semiconductor substrate exposed by the recessed portions. In this case, voids do not form in the vicinity of the transistor so that the transistor reliability is improved.
Moreover, preferably, the recessed portion forming step is performed inside an appropriate chamber followed by the epitaxial growth step performed inside the same chamber.
In this case, the cleanliness of the semiconductor substrate surface, particularly before the epitaxial growth step, is retained, allowing the stable growth of a silicon epitaxial growth layer.
The step of forming a pair of impurity regions preferably and specifically includes, after the recessed portion forming step, the step of introducing into the main surface of the semiconductor substrate an impurity of a prescribed conductivity type by ion implantation method. Particularly, during ion implantation, it is more desirable to introduce the impurity ions of the prescribed conductivity type into the main surface of the semiconductor substrate by oblique rotational ion implantation. In this case, the impurity ions may be easily introduced into the surface of the semiconductor substrate beneath the recessed portions.
Moreover, the step of forming a pair of impurity regions alternatively may involve, after forming the conductive layers, introducing the impurity into the surface of the semiconductor substrate through the conductive layers by ion implantation method.
Furthermore, the step of forming a pair of impurity regions may include the step of introducing the impurity of the prescribed conductivity type into the conductive layers, and thereafter, the step of forming the impurity regions by diffusing the impurity into the main surface of the semiconductor substrate. In this case, the impurity regions can be formed without difficulty.